For optoelectronic devices applied to transmitting or receiving optical signals or energies, much optical transmission between the device and the outside needs be realized through optical fibers (or in short “fibers” hereof), therefore, an optical coupling issue exists in these optoelectronic devices between a photoelectric conversion element, which is the key part of the device, and the optical fiber. This is called the fiber packaging of the optoelectronic devices.
Please refer to FIG. 1, which shows the working principle of the side-coupling optical fiber unit from under the prior art. That is, the input and the output of the light beam at the end of the optical fiber is not along the axial direction of the fiber but the radial direction, i.e., at a right angle to the fiber axis. The side coupling is realized through an inclined end facet formed on the optical fiber end 140, which satisfies a total-internal-reflection condition. The side-coupling structure is normally used in coupling of an optical fiber to a surface-type photoelectric conversion element 100 under a certain packaging mode, more commonly seen as between an optical fiber 110 and a surface-type semiconductor optical-receiver chip in a butterfly package.
Please refer to FIG. 2, which illustrates the typical embodiment of the above side-coupling optical fiber unit in the prior art. The fixing of the fiber 110 at a spatial position is realized through a generally metallic package 150 that contains the photoelectric conversion element 100. A circular through-hole 160 for passing the fiber is arranged in the package 150, and a certain section of the fiber 110 is fixed in the through-hole 160. A typical fixation process is to tightly attach a metal sleeve layer 170 to a naked fiber 110, whose outer protective materials have been removed, and to solder the fiber 110 with the attached metal sleeve layer 170 into the through-hole 160 by metallic solders, wherein the process forms an airtight and stable packaging.
In the side-coupling structure of the prior art, the optical fiber needs be placed above the surface-type photoelectric conversion element, due to the following two main reasons:
(1) The structure that the working (coupling) surface of the photoelectric conversion element is being upward and the fiber end is above the working surface is easy for observation and alignment. The effective working region of the working surface of the photoelectric conversion element is generally a limited round region 120 circled by a metallic ring electrode 130 (as shown in FIG. 1). The observation and the alignment are handled under a microscope.
(2) The lead electrode 130 of the photoelectric conversion element is generally situated on the same surface of the working surface of the element (as shown in FIG. 1), and electrical connections between this lead electrode and the electrode of the mounting substrate of the element, between electrodes of all elements, and between an electrode of an element and the package frame, are generally realized through wire bonds, the wire being generally an ultrathin gold wire and the bonds formed by professional wire bonding machines, wherein the wire bonding process requires that each electrode surface be in an upward position. This necessitates also the upward positioning of the working surface of the photoelectric conversion element.
In the prior art, although the structure that the optical fiber is positioned above the photoelectric conversion element is easy for observation and alignment, a stability issue of the fiber fixation comes into existence due to the way of fiber fixing. The problem begins to show up when the working region of the photoelectric conversion element becomes very small. For a semiconductor optoelectronic chip used in the optical receiving in the optical communication, the size of the working region directly affects the working speed of the chip, as the higher the speed, the smaller a designed working region is to be needed. In the prior art, for the fiber fixing is not directly made to the fiber end but to a certain limited section some distance away from it, the fiber end can have quite a large spatial freedom. Particularly, for a slight angle of tilting of the fixed section, a relatively large displacement can occur at the fiber end. Moreover, the fiber fixing utilizes metallic elements which are in nature with large thermal expansion and contraction effects, including the unevenly distributed metallic solder materials, wherein this metalized, soldering process is needed for an airtight packaging. Hence, the fiber end is easy to drift away from its original optimal coupling position, for a slight movement of the fixed section of the fiber resulting from stress variation in the soldering part due to such as the temperature change, which then leads to device performance degradation or even fail.
At the unit channel speed reaching 10 G (1 G=109) bits per second applied in current optical communication systems, the diameter of the working region of a semiconductor optoelectronic chip for the optical receiving has shrank to as small as 30 μm. As the speed increases to the next grade of 40 G, the diameter will further decrease to 12 μm, whereas the diameter of the light beam propagated in a conventional single-mode fiber is already about 10 μm. Therefore, with the increase of the speed, the optical coupling becomes more and more sensitive to the position shift of the fiber. In current 10 G-dominated industrial product developments and production practices, problems out of this coupling instability such as reliability hard to be achieved, low primary qualification rate, and too long manufacturing time have already been in existence, making a production difficult to enter high volume, together with a high cost.
On the other side, while the speed goes up to higher grades, requirements on the electrical packaging of the optoelectronic devices also grow. The impeding and parasitic effects brought by the bonding wire on high-frequency electrical signals become stronger with the increase of signal frequency, and to a certain stage the performance becomes remarkably deteriorated. In theory, the situation of frequency upgrade can be accommodated by reducing the wire length. However, due to limitation of the wire bonding process wherein the wire length can not be decreased permanently, when the signal wire reaches as short as 100 μm, the employing of the wire bonding technology becomes quite difficult. This situation basically corresponds to a wire length requirement when the speed reaches 40 Gb/s.
An effective solution is to adopt the flip-chip mounting technology, wherein the lead electrode of a chip element faces downward and is directly bonded to the electrical circuit on the submount or substrate by such as a commonly used soldering method with some solder materials. As such, the connection lengths are made at a minimum, and thereby it is able to meet the high-speed requirement. Meanwhile, the flip-chip structure also provides advantages as improvement in heat dissipation, increase of electrical connection density, etc. The flip-chip mounting is already an established process in the microelectronics field. As for an optoelectronic element, a main type being that the lead electrode and the working surface for the optical coupling are on the same surface, if the flip-chip technology is to be adopted, the working surface will need be positioned downward, which is incompatible with the conventional fiber packaging technology. Since the positioning and the fixing of the optical fiber are done with the through-hole structure of the package in the prior art, all fiber coupling processes need rely on this package. This makes the coupling of the optical fiber placed below a photoelectric conversion element whose working surface is positioned downward difficult to conduct, for the fiber is invisible in manipulation. As a result, the current fiber packaging technology has restricted the implementation of the flip-chip technology which otherwise could be adopted in many optoelectronic devices.
Invention Contents
This invention aims to provide a side-coupling optical fiber assembly and the fabrication method thereof for overcoming the defects described above in the prior art.
For this purpose, the invention first provides a side-coupling optical fiber assembly that comprises a first substrate, an optical fiber, and a second substrate, wherein at least one groove is formed on one surface of the first substrate, the optical fiber is arranged in the groove, and the second substrate is arranged above the first substrate, pressing and covering the optical fiber.
Among them, an inclined facet is formed on the fiber end positioned between the first and the second substrates. The inclined facet is to realize a total internal reflection of the light beam transmitted in the optical fiber.
The first and the second substrates are fixed together to ensure that the optical fiber between them loses all its degrees of freedom.
Preferably, one end edge facet of each of the first and the second substrates is in the same surface of the inclined end facet of the optical fiber.
A first light-passing hole is arranged in the second substrate, corresponding to the path of the light beam after it is totally reflected from the inclined end facet of the optical fiber; or, this second substrate is made of an identical or similar material as that of the waveguide of the optical fiber for reducing the internal reflection.
Preferably, the second substrate is an extended optical bench, and at least one optical element is installed on it.
The optical element installed on the second substrate can be a lens, and the transmission region of the lens corresponds to the projection region of the totally reflected light beam.
Preferably, the side-coupling optical fiber assembly includes a spacer plate. The spacer plate is arranged above the second substrate. A slot hole is arranged in the spacer plate and the lens is adapted to the slot hole.
Preferably, the side-coupling optical fiber assembly includes a mounting substrate of an optical receiving chip, a second light-passing hole, and an optical receiving chip, wherein the mounting substrate is arranged above the spacer plate, the second light-passing hole is arranged in the mounting substrate and corresponds to the projection region of the light beam after through the lens, and the optical receiving chip is arranged above the second light-passing hole and is combined with the mounting substrate.
Second, the invention provides a fabrication method of the side-coupling optical fiber assembly, which comprises the following steps:
an optical fiber is put into a groove formed on a first substrate;
the first substrate and a second substrate are fixed together by means of adhesion, which makes the optical fiber lose all its degrees of freedom;
the end face of the optical fiber and the end edge faces of the first and the second substrates are ground or polished into one same inclined surface, wherein a total internal reflection of the light beam in the optical fiber takes place at the formed inclined end facet of the fiber.
In accordance, there's a step to keep the first and the second substrates in a horizontal position after they are fixed together, as follows:
at least one additional optical fiber having the same diameter as the aforementioned optical fiber is put into another groove of the first substrate, after which the first and the second substrates are fixed together.
In comparison with the prior art, this invention has the following advantages:
first, as a side-coupling optical fiber unit, it provides a direct and complete confinement of the coupling optical fiber end, and in combination with a mounting substrate of the photoelectric conversion element, it is able to realize a stable and reliable lateral fiber coupling under rigorous requirements;
second, the side-coupling optical fiber assembly of the invention is flexible in installation, and it's convenient for accommodating various kinds of packaging technologies including the flip-chip mounting;
last, the side-coupling optical fiber assembly of the invention can provide a new assembling platform, upon which it can be extended to a function-versatile, all-in-one side-coupling optical fiber assembly including being implemented as a complete element mounting platform.